1. Field of the Invention
Generally, the present disclosure relates to the field of fabricating microstructures, such as integrated circuits, and, more particularly, to controlling critical dimensions (CD) in sophisticated imaging processes.
2. Description of the Related Art
The fabrication of microstructures, such as integrated circuits, requires tiny regions of precisely controlled size to be formed in one or more material layers of an appropriate substrate, such as a silicon substrate, a silicon-on-insulator (SOI) substrate or other suitable carrier materials. These tiny regions of precisely controlled size are generated by patterning the material layer by performing lithography, etch, implantation, deposition, oxidation processes and the like, wherein, typically, at least in a certain stage of the patterning process, a mask layer may be formed over the material layer(s) to be treated to define these tiny regions. Generally, a mask layer may consist of or may be formed by means of a layer of radiation sensitive material, such as photoresist, that is patterned by a lithographic process, typically a photolithography process. During the photolithography process, the radiation sensitive material or resist may be applied to the substrate surface and then selectively exposed to ultraviolet radiation through a corresponding lithography mask, such as a reticle, thereby imaging the reticle pattern into the resist layer to form a latent image therein. After developing the photoresist or any other radiation sensitive material, depending on the type of resist or radiation sensitive material used, positive resist or negative resist, the exposed portions or the non-exposed portions are removed to form the required pattern in the layer of photoresist or radiation sensitive material. Based on this resist pattern, actual device patterns may be formed by further manufacturing processes, such as etch, implantation, anneal processes and the like.
The immense progress in the field of semiconductor fabrication made over the last decades has been made possible essentially by the significant technical advance of the optical photolithography process technique, wherein significant progress in improving the resolution has led to a continuous reduction of critical features sizes, which are now well beyond 50 nm in sophisticated semiconductor devices. That is, an important factor in improving the resolution is the lithography process itself, in which patterns contained in a photomask or reticle are optically transferred to a substrate by means of an optical imaging system. Therefore, great efforts have been made to steadily improve the optical resolution of the imaging system, such as numerical aperture, depth of focus and wavelength of the light source used. As is well known, the resolution of an optical system is proportional to the wavelength of the light source used and to a process-related factor and is inversely proportional to the numerical aperture. For this reason, the wavelength may be reduced and/or the process-related factor may be reduced and/or the numerical aperture may be increased in an attempt to further improve the overall resolution. In recent decades, all three approaches have been concurrently taken, wherein, for instance, the exposure wavelength has been reduced from 436 to 365 nm and further to 248 nm towards 193 nm, which is the exposure wavelength currently used for critical lithography processes requiring critical dimensions that are well below the exposure wavelength. On the other hand, the depth of focus, i.e., the range within objects may be imaged with sufficient accuracy, is inversely proportional to the square of the numerical aperture so that significant efforts have also been made in this field. For example, starting with 0.25 in the early 1980's, the numerical aperture went up to 0.93 in 2004. In this situation, further improvements seemed difficult to be achieved until the technique of immersion photolithography was introduced and gave a new boost to further developments and increasing the numerical aperture above 1.0, wherein currently, via the steps of 1.07 and 1.2 in late 2007, a value of 1.35 has been achieved.
However, any further progress in reducing the exposure wavelength and/or increasing the numerical aperture may not be expected in the near future and, hence, any further reduction of the critical dimensions may require fully exploiting the capabilities of presently available complex lithography systems and associated process techniques. For example, even any subtle changes in the exposure dose within a single die region may result in a variance of the critical dimensions of several nanometers, which may no longer be acceptable in sophisticated semiconductor designs. Consequently, any critical factors, which may result in a variation of the critical dimensions, may have to be identified and appropriate corrective measures have to be taken in order to fully exploit the capabilities of presently available lithography techniques. For example, any non-uniformities of the imaging itself may be caused by certain imperfections during the manufacturing of the complex components, such as lens systems, illumination sources and the like. In advanced lithography tools, typically optical projection systems are provided which reduce the size of mask features formed in the reticle by a certain factor, for instance 5:1, 2:1 and the like, thereby providing significant advantages with respect to the quality of the masks, since the mask features may be formed on the basis of less critical dimensions. These projection systems typically comprise a plurality of lenses formed from two or more materials that may provide the desired transmission characteristics for the wavelength under consideration. Due to any imperfections during the manufacturing process, for instance with respect to the shaping of the individual lenses and due to imperfections in the materials used, however, a certain degree of deviation from an ideal imaging behavior is typically encountered, which may also be referred to as lens aberration. This non-ideal imagining behavior is typically quantitatively estimated after the manufacturing of the optical system and may also be monitored during the operation of the optical system, which may be accomplished by determining tool-specific characteristics, which quantitatively describe the discrepancy between an ideal wave front and the actual wave front of the imaging system under consideration. For this reason, a plurality of correction mechanisms are typically implemented into complex lithography systems, which may allow a certain re-adjustment of critical parameters, such as a correction of exposure dose and the like.
Also the lithography mask itself has been identified as a source of any non-uniformities during a complex lithography process, which may be caused by imperfections upon manufacturing a photomask. These imperfections may, for instance, be caused by imperfect materials and process techniques, while also a certain degree of degradation of the mask may occur over the lifetime of the photomask. For example, a plurality of degradation mechanisms have been identified, which may increasingly cause a non-uniform transmission, which may thus be superimposed with the basically non-uniform transmission caused by the specific pattern of mask features corresponding to a certain device level of a semiconductor device under consideration. That is, the very complex layout of modern semiconductor devices may result in complex patterns of circuit elements, wherein features of minimal dimensions, i.e., critical dimensions, may have to be provided with a certain density in some areas, while, in other areas, according to the layout of, for instance, a complex logic circuit, a different pattern density may have to be provided, which may generally result in a different global transmission behavior across the photomask, wherein, for instance, generally, a dense structure of mask features of critical dimensions may result in a reduced overall transmission, since a significant amount of light is lost due to the spatial filtering behavior of the lens aperture, when the mask features have dimensions at the resolution limit of the lens system.
Therefore, each imaging system may thus have its own fingerprint or signature due to the specific configuration of the imaging system and due to its individual imperfections in the illumination system and the lens systems of the lithography tool. Similarly, each lithography mask comprises a complex pattern of mask features, which, for instance, are typically formed on the basis of sophisticated correction techniques, such as OPC (Optical Proximity Correction) and the like, in order to correct pattern-specific deformations, which may occur in the actual resist features when performing a well-defined imaging process on the basis of a preselected imaging system. Consequently, in the OPC technique, the lithography process may be modeled with respect to a certain critical layout pattern for a given lithography process in order to appropriately redesign the critical mask features due to any predicted deformations of the basic layout that are predicted by the OPC model. Consequently, in addition to the complex pattern of mask features, which may include the OPC corrections, reticle-specific imperfections are typically convoluted over the pattern, thereby also imparting an individual fingerprint or signature to the photomask and the resist features obtained therefrom, which, in combination with the tool specific characteristics, may thus result in certain process non-uniformities, which may not be acceptable in very critical lithography processes that require, for instance, extremely uniform critical dimensions across the entire exposure field of the system. Furthermore, at least some of the factors contributing to the individual signatures of a mask/lithography tool pair may represent dynamic factors, so that corresponding correction strategies may have to be applied in a dynamic manner so as to maintain long-term stability of critical lithography processes. Consequently, in the last years, complex correction strategies have been developed in which a certain degree of correction may be applied in a controllable manner when performing critical lithography processes. For example, in some APC (Advanced Process Control) strategies, a sensitive adjustment of exposure dose values may be applied on the basis of measurement data obtained for each individual reticle/lithography tool combination, in order to take into account the associated signature.
Generally, the calculation of accurate correction functions requires the determination of the signature in a very precise manner in the first place. A frequently applied possibility to determine the signature of a lithography mask is to measure the critical dimensions of the mask features on mask level by appropriate SEM (Scanning Electron Microscopy) procedures. For this purpose, specifically designed test patterns of specific configuration are incorporated into the circuit layout, i.e., into the complex pattern of mask features, in order to determine mask-specific characteristics on the basis of the plurality of dedicated test patterns. For example, a plurality of mask patterns of the same configuration may be distributed over the entire area of the lithography mask and specific characteristics, such as lateral dimensions, material non-uniformities and the like, may be determined, wherein it is assumed that characteristics obtained from the distributed mask patterns may also appropriately represent any imperfections and non-uniformities in the actual circuit mask features. These well-established conventional strategies may thus provide information about discrete positions of the mask, however, without providing any information with respect to any non-uniformities that may be introduced by the exposure tool itself, i.e., by the lens system and the illumination system. Hence, upon establishing a correction function that is based on dedicated test patterns in the lithography mask, the tool-specific non-uniformities may not be taken into consideration. Moreover, due to the limited number of dedicated test patterns in the lithography mask, a corresponding map of the two-dimensional discrete function obtained on the basis of the test pattern may not necessarily represent the remaining mask areas with appropriate precision. For example, any high frequency variations, i.e., spatial high frequency variations, of real circuit patterns in the lithography mask are typically not included in the discrete two-dimensional function and any values interpolated therefrom. Hence, performance of the correction strategy based on discrete test patterns may be very limited. On the other hand, the measuring of real circuit patterns is, however, not a very promising approach, as these patterns are usually established on the basis of dedicated OPC models to correct each critical individual pattern in view of the limited optical transfer function of the lens of the imaging system under consideration. It is thus very difficult to differentiate between any “non-uniformities” introduced by the OPC correction and any other non-uniformities, which may be caused by other factors, such as manufacturing imperfections and the like.
Therefore, in other approaches, appropriate algorithms are being established for mask inspection tools in order to appropriately analyze the measurement data gathered during an inspection process in order to determine mask-specific non-uniformities. In a mask inspection process, typically, the lithography mask may be used during an imaging process, in which an appropriate light beam is passed through the mask and an appropriate imaging system, which will typically be configured so as to obtain the desired information, for instance with respect to transmission characteristics and the like, wherein, typically, the lens system and the illumination system may be optimized with respect to mask inspection processes and thus these components may differ from any such components in actual lithography tools, for instance in terms of exposure wavelength, numerical aperture and the like. Furthermore, it is quite questionable whether any such algorithms based on optical mask inspection data may enable a precise determination of the signature of the individual lithography masks, in particular if lithography masks for sophisticated logic circuitry are considered, in which significantly varying pattern densities and different sizes have to be implemented in a single lithography layer, for instance compared to layouts of memory devices, in which nearly identical patterns may be repeated many times. In particular, the reliability of any such algorithms may be inferior, since typically, as explained above, dedicated optical proximity corrections may be provided in a pattern-specific manner, wherein the optical “response” of the optical proximity correction may be specifically designed for well-defined imaging conditions of the photomask, which are not present in the optical inspection tool. In particular, any such algorithms based on the determination of the transmission characteristics may be heavily affected by the significant variance of pattern density and sizes of the mask features in combination with the dedicated optical proximity corrections. Furthermore, in these approaches, only the mask-specific non-uniformities are taken account of, while the lithography tool-specific imperfections are not included.
Accordingly, in other approaches, measurements on substrate basis may be performed by using SEM tools so that, in this case, performance of the lithography process, including mask non-uniformities, the signature of the lens distortion and the illumination of the lithography tool, is taken into consideration. On the other hand, the measurements are taken from specifically designed test patterns, as described above, and hence the authenticity of the measurement data may be inferior, since the characteristics of critical circuit patterns may not be appropriately reflected by the test patterns. It could be contemplated to obtain measurement values from thousands of actual circuit patterns, which, however, requires significant effort in terms of process time and inspection tool resources, thereby rendering this approach less attractive. Moreover, also in many cases, the critical dimensions and the local neighborhood of critical circuit patterns may significantly change in complex circuit patterns of logic circuitry, which may thus finally result in non-representative measurement results, which in turn may lead to an inappropriate correction function.
The present disclosure is directed to various methods and devices that may avoid, or at least reduce, the effects of one or more of the problems identified above.